FUMIGATION OF CITRUS TREES. 27 



figure lb. The second step in the procedure is shown in figure 2 of 

 Plate VI, while the remaining steps are the same as in Plate V, 

 figures 4, 5, and 6. 



In covering very large trees derricks of the nature described on 

 page 21 are used. Four men are required for their manipulation, 

 which is shown in Plate VII. A derrick is raised to a nearly upright 

 position at each side of the tree (Plate VII, figs. 1-3), leaning at a 

 slight angle backward and held in this position by the guy-rope 

 attached to its top (Plate VII, fig. 3). The movable pulley of each 

 derrick is then attached to a ring in the tent (Plate VII, fig. 3) and 

 pulled up to the top of the derrick, where it is held (Plate VII, fig. 4). 

 By pulling on- the guy-rope the derrick is caused to fall forw r ard, 

 drawing the tent over the tree. 



CALCULATING THE DOSAGE. 



Having covered the trees, the next requirement is the amount of 

 chemicals to use, or the dosage. The dosage is the most important 

 consideration in the gas process. It varies not only with the size of 

 the tree but also with the character of insect to be destroyed. Spe- 

 cific recommendations of dosage for the principal insects injurious 

 to citrus trees in California are given elsewhere in this bulletin 

 (pp. 51-61). 



The first requirement in calculating the dosage for a tree is to 

 compute the cubic contents inclosed by the tent when in position 

 over the tree. Although most citrus trees possess a certain general 

 similarity in shape, they are nevertheless somewhat irregular, no two 

 ever being identical in all respects. This irregularity renders it 

 impracticable to determine the contents to within a cubic foot or so 

 of its actual volume; yet it can be approximated with a sufficient 

 degree of accuracy for such practical work as fumigation. In order 

 to calculate the cubic contents of an object it must be considered as 

 shaped like some regular geometrical figure or figures. The figure 

 which most closely approximates in shape an orange or lemon tree 

 before it has been pruned is a cylinder surmounted by a hemisphere, 

 and in computing the volume of citrus trees the3 T are considered to 

 be of this shape. 



If the height and width of a tree covered with a tent is known, it 

 is a comparatively simple matter to calculate its contents. In the 

 past work in California the dosage has been based upon these two 

 measurements. After a tree is covered with a tent it is a matter of 

 some difficulty to determine its height and width. By using as fac- 

 tors the distance around the bottom of the tent and the longest dis- 

 tance over the top of the tent we arrive at a more practicable method 

 by which to compute the cubic contents of a given tree. Using these 



